This invention relates to a hydraulic suspension strut for use in heavy vehicles.
Suspension struts are used in many types of vehicles to absorb and dampen transient forces that a vehicle is subjected to as it travels over terrain. In a typical suspension strut, there are a plurality of cavities that contain a viscous fluid such as hydraulic fluid (oil). As the strut is compressed, fluid is allowed to flow between the cavities through orifices of varying sizes. The viscous fluid flowing through the orifices provides damping within the strut. The amount of damping within the strut can be adapted to the types of transient loads expected by increasing or decreasing the diameters of the orifices through which the fluid flows and by changing the viscosity of the fluid itself.
It is known in the art that as a suspension strut reaches its full extension, harmful metal to metal contact may occur between the piston and piston guide in addition to large spike loads being transferred to the vehicle at the strut attachment points. This situation results in unnecessary component wear which will decrease the operating life of the strut. To avoid this situation, it is known in the art to increase the damping within the strut to slow the rate of expansion as the strut nears full extension. For this purpose, mechanical rebound stops are sometimes provided which are attached to the outside of the piston rod and contractible within the rod guide.
Generally, mechanical rebound stops consist of a resilient bumper or a piston/chamber arrangement. As the bumper compresses, it absorbs energy that would otherwise be dissipated by the contact of metal components within the strut thus avoiding unnecessary wear. The bumpers are inexpensive but have limited energy storage/dissipation ability. The piston/chamber arrangement relies on expensive machining and surface treatment processes to achieve higher levels of energy dissipation, and is not cost-effective.
This invention in a preferred embodiment provides a hydraulic strut which can be made in a relatively inexpensive manner and has a sliding seal that can increase the damping rate within the strut as the strut approaches full extension.
In one aspect the invention provides a suspension strut comprising:
(a) an outer cylinder having an inner surface;
(b) an inner cylinder defining a first space therein, said inner cylinder being slidable in said outer cylinder and defining an annular second space between said inner cylinder and said outer cylinder;
(c) said inner cylinder including a cap assembly defining a third space between said cap assembly and said outer cylinder;
(d) at least one valve on said inner cylinder to permit fluid flow between said annular second space and said third space; and
(e) a shutoff attached to the inner surface of said outer cylinder and cooperating with said valve to effect a gradual shutoff of fluid flow from said annular second space through said valve to said third space as said inner and outer cylinders are extended with respect to each other.
Further aspects and advantages of the invention will appear from the following disclosure, taken together with the accompanying drawings.